Nonconductive coatings are applied at intersections of materials, typically in the form of a fillet seal for purposes of enclosing and sealing the intersection. Often these coatings are required to attain a minimum thickness. In many cases, these coatings can shrink by as much as 20% as they dry. As such, what appears to be an adequate coating thickness may later shrink to less than a minimum thickness. In the aerospace industry, quality assurance programs have been developed to inspect coatings and fillet seals on aircraft to ensure that they achieve minimum desired thicknesses.
For example, one sealant, BMS5-26, Type II, Integral Fuel Tank Sealant, is used to coat intersections of an airplane wing. In many aircraft, the wing is also the support structure for an internal bladder fuel tank, and it is therefore important to seal intersections and gaps. The sealant must be applied to any intersection to prevent a leak path. The sealant is typically applied to the border of all electrical pass-through brackets, all fasteners, and along the faying surface of the web and skin. The greatest amount of sealant is applied along the intersection of the heavy longerons and the wing skin.
When inspecting the sealant coatings at these intersections, quality assurance inspectors attempt to determine the minimum thickness of a coating. Unless coupled with a destructive test, nondestructive visual inspections are often not adequate. One test requires removing the sealant and physically measuring the thickness, from the radius of a concave fillet to the intersection of the web and skin. The physical measurement of the destroyed seal may then provide a rough visual reference for performing a visual inspection of the remaining sealant. These tests are based upon the visual perception of the inspector.
This procedure has several drawbacks. For one, visual inspections introduce subjective reproducibility errors. Also, the destroyed section must be repaired by having a coating seal reapplied. Consequently, the repaired area is not as structurally contiguous as the original seal. Also, additional labor is required to perform the destructive test and subsequently reapply the sealant. In the case of hard to reach areas, visual assessment is made based on experience. When an inspector is not convinced that a proper amount of sealant has been applied, they may insist that sealant be reapplied. Reapplication of sealant, therefore, also increases labor.
As such, there has not been an adequate non-destructive method for determining minimum coating thickness requirements. Therefore, there is a need in the art for a portable nondestructive coating thickness measurement gauge capable of measuring coating thicknesses at intersections, such as a fillet seal.